Solid polymer electrolytes or films have been well known in the art for many years. These polymers are typically characterized by high ionic conductivity, herein defined as greater than 1×10−6 S/cm. Such high conductivity values make them valuable where rapid transport of ionic species, for example, protons, is useful, for example in fuel cells. Additionally, it is desirable for such ionically conducting polymers to be made in the form of membranes or thin films. In so doing, the resistance to ionic transport, which is a function of the film thickness, can be reduced. These materials must also function in the temperature range of interest, which can vary from below room temperature up to a large fraction of the melting temperature of the polymer, depending on the application. Additionally, the polymer must be robust mechanically, so that it does not crack, either during installation in a fuel cell, or during use.
The use of ionomers as solid polymer electrolytes in fuel cells is well known, having been developed in the 1960s for the US Gemini space program. Historically, the industry has moved from phenol sulfonic materials, which suffered from poor mechanical and chemical stability; to polystyrene sulfonic acid polymers, which have improved mechanical stability, but still suffer chemical degradation; to poly(trifluoro-styrene)sulfonic acid, which has improved chemical stability, but poor mechanical stability; to perfluorinated sulfonic acid materials (commercially available as NAFION® membranes), which has improved mechanical and chemical stability [e.g., see A. J. Appleby and F. R. Foulkes, Fuel Cell Handbook, Van Nostrand Reinhold, New York, 1989; Table 10-1, pg. 268].
The perfluorinated sulfonic acid materials, for example those disclosed in U.S. Pat. No. 3,282,875, U.S. Pat. No. 4,358,545 and U.S. Pat. No. 4,940,525, are still far from ideal ionomers. These materials must be hydrated to conduct protons at an acceptable rate. As a result, in dry conditions or at temperatures above 100 degrees C., they work poorly in hydrogen-oxygen or hydrogen-air fuel cells. Furthermore, these fluoropolymer ionomers are expensive to produce because of the inherently high cost of the fluorinated monomers required for their preparation. Finally, as more fully described below, they tend to have a high permeability to methanol, and therefore are inefficient electrolytes for use in direct methanol fuel cells.
These limitations have led to the development of several classes of ionomers that are not substantially fluorinated, but rather are based upon aromatic or linear polymers. In U.S. Pat. No. 4,083,768 a polyelectrolyte membrane is prepared from a preswollen membrane containing an insoluable cross-linked aromatic polymer. Although these membranes have low ionic resistance, the controlled penetration of the functional groups during preparation can make preparation difficult. In U.S. Pat. No. 5,525,436, U.S. Pat. No. 6,025,085 and U.S. Pat. No. 6,099,988 the preparation and use of polybenzimidizole membranes as ionomers is disclosed. These polymers are described as particularly suitable for use at temperatures above 100 degrees C. U.S. Pat. No. 6,087,031 discloses a ionomer comprising a sulfonated polyethersulfone that is suitable for use in a fuel cell. Kono et. al. discloses in U.S. Pat. No. 6,399,254 a solid electrolyte having a reduced amount of non-cross-linked monomer that can be rapidly cured through exposure to active radiation and/or heat and has high conductivity. Finally, Wang et. al. have disclosed in WO 0225764 ionomers made by direct polymerization of sulfonated polysulfones or polyimide polymers.
Other substantially non-fluorinated ionomers are those in the class of sulfonated Poly(aryl ether ketone)s. It is convenient to prepare sulfonated poly(aryl ether ketone)s by post-sulfonation. However, post-sulfonation results in the placement of the sulfonic acid group ortho to the activated aromatic ether linkage, where the sulfonate groups are relatively easy to hydrolyze. Moreover, only one sulfonic acid per repeat unit can be achieved. To overcome this limitation, a new route was developed to prepare sulfonated poly(aryl ether sulfone)s with monomer containing sulfonate groups derived from sulfonating the dihalide monomer. Sulfonation of the dihalide monomer, 4,4′-dihalobenzophenone and 4,4′-dihalodiphenylsulfone, results in sulfonic acid functionalization on both deactivated phenyl rings ortho to the halogen moiety, which offers them more chemical stability against desulfonation, and allows for two sulfonic acid groups per repeat unit of the resulting polymer. This approach displays other advantages, including being free from any degradation and cross-linking, and the ability to easily control the content of sulfonate groups by adjusting the ratios of the dihalide monomer to the sulfonated dihalide monomer.
In order to prepare ion exchange membrane for polymer electrolyte fuel cells (PEMFC) with excellent combined physical chemical properties and of low cost, attempts have been made by utilizing sulfonated poly(aromatic ether sulfone) and sulfonated poly(aromatic ether ketone). These membranes can be made by two methods. One is direct sulfonation of polymers, as reported in Polym. V28, P1009 (1987) wherein direct sulfonation of poly(aromatic ether ketone) to prepare sulfonated poly(aromatic ether ketone) was reported. This method is straightforward, but decomposition and crosslinking also occurred. The degree of sulfonation is also difficult to control. In addition, the sulfonated group directly attached to bisphenol-A could cause sulfonated group be hydrolyzed and detached from the polymer structure after prolong service at high temperature. Another method is to prepare sulfonated monomers first, followed by polymerization afterwards. Macrom. Chem. Phy., (1997), P1421 (1998) reported this method. First, sulfonated difluoro-benzophenone was obtained by sulfonation of difluoro-benzophenone, then it was mixed with some difluoro-benzophenone and bisphenol-A, followed by a copolymerization into sulfonated poly(aromatic ether ketone). The polymer structure is characterized by the following structure:
The polymerization process does not induce decomposition or crosslinking side reactions and it could control degree of sulfonation. The sulfonic acid groups located on the aromatic ring structure derived from benzophenone are much more stable than the ones on bisphenol-A rings. However, because of the existence of methyl group in the polymer structure, its anti-oxidation property is reduced. Furthermore, as the content of sulfonated group increases, swelling in water becomes very severe.
Recently, bisphenol A-based and phenolphthalein-based and 4,4′-thiodiphenol-based sulfonated poly(aryl ether ketone)s were prepared by another method. Generally, most homogeneous ionomers have the problem of large swelling degree at compositions where they have reasonable conductivity. So other components are blended with the polymer in order to obtain ionomer membranes with lower swelling.
One issue with many of these polymers is that the ionic conductivity is not as high as desirable. A high ionic conductivity in an ionomer is desirable because the higher the ionic conductivity, the lower the cell resistance when the polymer is used as the electrolyte in a fuel cell. Because lower cell resistance leads to higher fuel cell efficiency, lower resistance (or high conductance) is better. One approach to reducing the resistance of these ionomers is to reduce its thickness, as the resistance is directly proportional to thickness. Unfortunately, these ionomers cannot be made too thin because as they become thinner, they become more susceptible to physical or chemical damage, either during cell assembly or cell operation. One approach to deal with this issue has been disclosed in RE 37,707, RE 37,756 and RE 37,701 where ultra-thin composite membranes comprising expanded polytetrafluoroethylene and an ion exchange material impregnated throughout the membrane are disclosed. Composite ionomer membranes are also disclosed in U.S. Pat. No. 6,258,861.
One further complication in the use of ionomers as solid polymer electrolytes in fuel cells is the need for the electrolyte to act as an impermeable barrier to the fuel. Should the fuel permeate through the electrolyte it reduces cell efficiency because the fuel that permeates through the electrolyte is either swept away into the outlet gas stream or chemically reacts on the oxidant side, giving rise to a mixed potential electrode. In either case, the fuel is not used for producing electricity. Furthermore, the fuel that permeates through the electrolyte may also poison the catalyst on the oxidizing side, further reducing the cell efficiency. This issue, called fuel crossover, is a particular problem when methanol is the fuel.
Methanol crossover rates tend to be high in many solid polymer electrolytes because the methanol absorbs and permeates in the polymer in much the same way that water molecules do. Since many solid polymer electrolytes transport water easily, they also tend to transport methanol easily. One approach to reducing methanol crossover is to simply use thicker membranes because the methanol transport-resistance (as defined below) increases with increasing thickness. This solution has limited utility, though, because as the thickness increases the ionic resistance of the membrane increases as well. Higher ionic resistance in the membrane is detrimental to fuel cell efficiency because it results in higher internal resistance and thus higher (iR) power losses. Therefore, the ideal membrane for direct methanol fuel cells would be one that has both very high methanol transport resistance and at the same time, has low ionic resistance. The combination of these two characteristics would allow the use of thicker membranes, leading to low iR power loss due to membrane resistance, while simultaneously minimizing the effect of methanol crossover.
The use of various polymers has been suggested to circumvent this methanol crossover issue. In WO 96/13872 the use of polybenzimidazole is suggested for direct methanol fuel cells. In WO 98/22989, a polymer electrolyte membrane composed of polystyrene sulfonic acid (PSSA) and poly(vinylidene fluoride) (PVDF) is reported to have low methanol crossover. In WO 00/77874 and U.S. Pat. No. 6,365,294 sulfonated polyphosphazene-based polymers are proposed as suitable ionomers for direct methanol fuel cells. Finally, poly(arylene ether sulfone) has been reported to have low methanol permeability [Y. S. Kim, F. Wang, M. Hickner, T. A. Zawodinski, and J. E. McGrath, Abstract No. 182, The Electrochemical Society Meeting Abstracts, Vol. 2002-1, The Electrochemical Society, Pennington, N.J., 2002]. Despite these attempts, a need still exists for an ionomer with lower methanol crossover rates and acceptably high ionic conductivity.
It is thus an object of this invention to satisfy the long-felt need for improved ionomers for use as a polymer electrolyte membrane and as an electrode component in fuel cells. It is also an object of the present invention to provide an improved method of forming a fuel cell using the inventive polymers. It is a further object of the invention to form a composite solid polymer electrolyte with improved properties comprising the inventive polymers and a support. It is yet another object of the invention to improve performance of a direct methanol fuel cell comprising the inventive polymers. Finally, it is also an object of the new invention to provide a fuel cell wherein the electrode comprises the inventive polymer.